1. Field of the Invention
The present invention relates to polarization-induced signal fading in interferometric fiber-optic sensors and particularly to an apparatus and method for minimizing such polarization-induced signal fading in an interferometric fiber-optic sensor by interrogating the sensor using an input polarization-modulated source. This allows the sensor output to be interrogated simultaneously at different input states of polarization (SOPs), or for the input polarization to be changed electronically by changing the point at which the output is sampled in relation to the input SOP modulation.
2. Background Description
In recent years interferometric fiber-optic sensors have been configured to detect a wide range of physical fields, such as acoustic waves and temperature fluctuations. These sensors typically operate with high sensitivity, good linearity, and over a wide dynamic range.
In operation, an interferometric fiber-optic sensor combines the light beams from two optical paths (through signal and reference arms) in the interferometer of the sensor and causes them to interfere to produce an interference pattern that is proportional to the field being sensed by the sensor. The interference between the light beams from the two paths can fade due to polarization drifts. If the states of polarizations of the two interfering beams from the two arms of the interferometer are co-directional, the interference is at a maximum and the sensitivity of the sensor is greatest. However, if the states of polarizations of the beams from the two arms are orthogonal to each other, the light from the two arms do not interfere and the sensitivity of the sensor is zero. Usually, conventional low-birefringence single-mode optical fiber is used to construct interferometric sensors of the Mach-Zehnder and Michelson types. As a result, random fluctuations in the state of polarization (SOP) of the interfering beams guided in the nominally circular fiber core can lead to fading of the interference signal. This phenomenon, termed polarization-induced (signal) fading' is well know. It should be noted at this time that polarization-induced signal fadding in interferometric fiber-optic sensors can occur due to two sources of polarization wandering-that in the actual arms of the sensor itself and that occurring in the input fiber to the sensor.
Several techniques have been proposed to overcome the effect of polarization-induced signal fading. These techniques range from the use of simple manual or automatic polarization controllers in the arms of the fiber interferometer, to techniques which can be described as polarization diversity detection schemes based on output polarization state selection.
In the case of manual or automatic polarization controllers, active polarization controllers, such as those based on fiber squeezers, Faraday rotators, or rotatable fiber wave plates developed for use in coherent communications systems, can be inserted into one or both of the fiber arms and used to match the SOP's of the interfering beams. This approach is well suited for use in laboratory sensor systems, but is not a practical solution for a prototype sensor, since such a sensor is usually required to operate passively and be remotely located from the source and detection/demodulation electronics.
In the case of polarization diversity detection, fading can be overcome by appropriately selecting a polarization mode at the output of the interferometer. This technique has been demonstrated using a passive three-axis polarizer mask at the interferometer output, and by effectively selecting the output polarization mode (general ellliptical) using a fixed output polarizer and active birefringence manipulation in the output fiber lead. This technique is demonstrated in U.S. Pat. No. 4,653,915 issued to Frigo et al. on Mar. 31, 1987.
Another technique is to construct the interferometer using high-birefringence (polarization preserving) fiber. However, due to the lack of high quality polarization preserving fiber components, notably directional couplers, this approach has met with little success. In a recent Patent Application, Ser. No. 07/301,301 filed on Jan. 25, 1989, titled "Apparatus And Method For Minimizing Polarization Induced Signal Fading In An Interferometer Fiber-Optic Sensor Using Input-Polarization Control" by A. D. Kersey, a method of overcoming signal fading through active feedback control of the input SOP is disclosed. The current invention discloses a novel technique for overcoming polarization induced signal fading based on an input-polarization diversity mode of operation.